Metro Dense Wave Division Multiplexed (DWDM) systems are evolving from ring-based topologies to mesh-based topologies as bandwidth requirements continue to grow. Wavelength selective switches (WSSs) are considered a key optical component to realize a high-degree reconfigurable optical add-drop multiplexer (ROADM) node. WSSs are used in DWDM networks to provide dynamic switching on a per-wavelength or a per-group of wavelengths level. WSSs can be used at network ingress, egress, or both to route/collect the traffic to/from different network directions. As described herein, network directions are referred to as degrees. For example, a two-degree node has two network directions, e.g. east and west, and a four-degree node has four network directions, e.g. north, south, east, and west. Generally, DWDM networks can include multiple nodes of varying degrees. However, considering the WSS is an expensive optical component, usually WSSs are implemented only at either the ingress or the egress of the node and an optical splitter or combiner is used at the other end instead.
Referring to FIG. 1, a common structure is illustrated for a conventional four-degree node 100 that may be used in a DWDM network. The node 100 includes a north direction 102, a south direction 104, a west direction 106, and an east direction 108. Each direction 102, 104, 106, 108 provides ingress/egress for each wavelength bi-directionally. Each of the directions 102, 104, 106, 108 is also interconnected to one another through a plurality of connections 110. The connections 110 enable a mesh architecture at the node 100 allowing any wavelength to be directed from any degree to any other degree, i.e. optical pass-through at the node 100.
As described herein, the node 100 includes a WSS 112 at each ingress for each direction 102, 104, 106, 108. The WSS 112 can include a 1×5, 1×9, or the like device. To indicate device fan out, WSSs 112 are often classified as “1×N” devices, with a “1×9” WSS meaning a 10 port device, with one common input and nine output ports. Here, the WSS 112 is illustrated with one input port (from the ingress), three drop ports for local add/drop, and three ports each connected to other directions. Accordingly, to scale the node 100, the WSS 112 must include additional ports to connect to the new directions. The egress direction for each direction 102, 104, 106, 108 includes a combiner and post amp 114. The combiner is configured to multiplex locally added wavelengths and wavelengths from each of the other directions, and the post amp provides optical amplification.
When the network degree of the node 100 scales above four, assuming two to four common add/drop ports are planned as is common, the coupler's insertion loss (IL) can be even larger than 10 dB, which requires a high gain post-amplifier to compensate intra-node loss. This introduces additional noise into the path. The equivalent noise figure (NF) of the combiner and post amp 114 can be simplified as the insertion loss plus the NF of the post amp. For every 3 dB additional combiner loss, the additional noise added will be doubled. Eventually, this will impact system reach and limit the number of degrees the node 100 can be scaled to and the reach between nodes 100. Referring to FIG. 2, a graph 200 illustrates an equivalent span penalty 202 with different combiner losses 204 as a function of span loss 206. To improve system reach, the combiner loss has to be reduced to reduce the equivalent span penalty and limit the degrees of the node 100. The combiner and post amp 114 could be replaced with a symmetrically configured WSS, but this adds another WSS 114 for each direction which significantly increases cost and network complexity.